gray card - 12 or 18%?

Started Jan 12, 2011 | Discussions
EdW66 Forum Member • Posts: 80
gray card - 12 or 18%?

I've been using a gray card from the back of Scott Kelby's lightroon 3 card. It does a reasonable job, but I find skin tones look a little cool and slightly toward green when using my strobes.

I've been looking at the pop up gray cards from lastolite and notice they have introduced a 12% gray card (ezybalance) in addition to the 18 percent. I understand that the 12% moves the midtones closer to the middle of the histogram, but what does this mean in real terms? does this effect the overall WB, or just the exposure? and is one better for one brand of camera than the other? (I use a Nikon D700)
Thanks in advance!

Onetrack97 Senior Member • Posts: 1,953
Re: gray card - 12 or 18%?

If you're using it for white balance, it doesn't matter what % of gray it is. What matters is that it has equal amounts of RGB.

You can use WhiBal, GreytagMacBeth Colorchecker, etc.

In fact, at the bottom of the color checker, you will see a row of black, several grays, and white. You will get the same color temp from all of the grays.

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OP EdW66 Forum Member • Posts: 80
Re: gray card - 12 or 18%?

thanks, that makes sense about the colour balance. Any one know if there's any other advantage to using 12% over 18%?

acsmith Senior Member • Posts: 1,850
Which yields the most correct exposure?

If you're using a gray card for just exposure which one delivers the most correct exposure with your equipment?

In essence the gray card is representing an average subject (meter calibration point) so that you can use it as a substitute when your subjects may be non-standard or variable so one or the other should yield better exposures. An 18% results in overexposure on one of my cameras but I haven't tested my newest camera.

A. C.

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christopherbarran Contributing Member • Posts: 927
Re: gray card - 12 or 18%?

I use a ezybalance from lastolite.
I must say I think my shots have improved quite a bit since using it.

I use them for WB. But apparently you can use them to get a good exposure too. I looked for and can't find the little booklet that came with the card that explains how to do this.

So I do have a quick question. Say am doing outdoor portraits. I normally have the model hold the ezybalance and I take a test shot (with flash firing etc) to get the right WB in addition to using the custom WB setting on my camera.
Can I do this to get the right exposure too?
Like suppose I want to do some off camera flash?

My way is to meter, then underexpose by say 1.7 stops, and use the flash to light correctly (by judging with my eyes and the camera review)

Can I have the model hold up the ezybalance and meter off the ezybalance and compensate as I normally do (-1.7 stops) then add the flash output to make it look right?

EdW66 wrote:

I've been using a gray card from the back of Scott Kelby's lightroon 3 card. It does a reasonable job, but I find skin tones look a little cool and slightly toward green when using my strobes.

I've been looking at the pop up gray cards from lastolite and notice they have introduced a 12% gray card (ezybalance) in addition to the 18 percent. I understand that the 12% moves the midtones closer to the middle of the histogram, but what does this mean in real terms? does this effect the overall WB, or just the exposure? and is one better for one brand of camera than the other? (I use a Nikon D700)
Thanks in advance!

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Fantasy Photo Senior Member • Posts: 1,398
Each Has an Advantage Over the Other

A little history: The 18% gray card was popularized by Kodak for use in the printing trade. A reflectance of 18% is a "middle gray" in the printing industry and the card allowed the press operator to insure the press was printing at the proper density. The card was gray, so it also helped insure the absence of any color tint in the final press run. Photographers saw the same value in using such a card for setting exposure and insuring accurate colors and they began using the 18% gray card as well. For photographers, the reflectivity of an "average" scene is between 12 and 13 percent. So, an 18% gray card would cause under exposure if used directly to set the exposure. But, by holding the card at a 45 degree angle to the camera, the reflectivity of the card would become about 12.7% since the reflectivity changes with the cosine of the angle (cos 45 is about 0.7 and 0.7 times 18 is 12.7). Or, just simple hold the card flat to the camera and then adjust the camera to over expose half a stop from what the reading from the card indicates. There were a few years when Kodak sold cards without instructions explaining (as I just did) how to use an 18% gray card to properly set a camera's exposure. That caused a lot of confusion among new photographers who didn't understand the concept behind the 18% gray card. For what it's worth, Kodak no longer manufactures the Kodak 18% gray card even though it still carries their name. I belive the owners of Tamron now have the rights to manufacture the card.

Advantage of an 18% gray card: With modern digital cameras, photographing such a card will produce a spike dead center in the camera's histogram if the camera's exposure is accurately set. (That's because an 18% gray is a "middle" gray.) You can also adjust the "exposure" during post processing by moving the spike to the center of the software's histogram during post processing. Digital shooters will probably find this card the best to use.

Advantage of a 12% gray card: A meter reading directly off the card will allow an accurate exposure to be set directly on the camera. Film shooters would probably find this card the best to use since they don't have a histogram to look at.

http://www.fantasy-photo.com

michaeladawson Forum Pro • Posts: 12,914
In theory, yes

Be aware that there is margin of error in the phrase "you should get the same color temperature from all of the gray squares."

My experience with the gray squares on the Color Checker chart is that you will get a color temp that is plus or minus about 100 degrees K. In other words, a difference of almost 200 degrees. I also consistently see a trend as you go from the lightest square to the darkest square. I forget if the temperature and tint rises or falls with the gray tone but it does change.

200 degrees may be within the expected margin of error.
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Dave George Regular Member • Posts: 433
Re: Each Has an Advantage Over the Other

Fantasy Photo wrote:

A little history: The 18% gray card was popularized by Kodak for use in the printing trade. A reflectance of 18% is a "middle gray" in the printing industry and the card allowed the press operator to insure the press was printing at the proper density. The card was gray, so it also helped insure the absence of any color tint in the final press run. Photographers saw the same value in using such a card for setting exposure and insuring accurate colors and they began using the 18% gray card as well. For photographers, the reflectivity of an "average" scene is between 12 and 13 percent. So, an 18% gray card would cause under exposure if used directly to set the exposure. But, by holding the card at a 45 degree angle to the camera, the reflectivity of the card would become about 12.7% since the reflectivity changes with the cosine of the angle (cos 45 is about 0.7 and 0.7 times 18 is 12.7). Or, just simple hold the card flat to the camera and then adjust the camera to over expose half a stop from what the reading from the card indicates. There were a few years when Kodak sold cards without instructions explaining (as I just did) how to use an 18% gray card to properly set a camera's exposure. That caused a lot of confusion among new photographers who didn't understand the concept behind the 18% gray card. For what it's worth, Kodak no longer manufactures the Kodak 18% gray card even though it still carries their name. I belive the owners of Tamron now have the rights to manufacture the card.

Advantage of an 18% gray card: With modern digital cameras, photographing such a card will produce a spike dead center in the camera's histogram if the camera's exposure is accurately set. (That's because an 18% gray is a "middle" gray.) You can also adjust the "exposure" during post processing by moving the spike to the center of the software's histogram during post processing. Digital shooters will probably find this card the best to use.

Advantage of a 12% gray card: A meter reading directly off the card will allow an accurate exposure to be set directly on the camera. Film shooters would probably find this card the best to use since they don't have a histogram to look at.

http://www.fantasy-photo.com

Thanks for posting that!

I have been using a lastolite 18% card for a few months now and a very helpful forum member over on the Nikon D90 forum helped with my understanding of what was going on with the 12% 18% thing, but reading your post was also most informative.

I find with my d90 that spot metering with a permanent meter bias to +2/6 ev results in a central spike so looks like I have the hang of it

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WFulton Senior Member • Posts: 2,698
Re: Each Has an Advantage Over the Other

Fantasy Photo wrote:

Advantage of an 18% gray card: With modern digital cameras, photographing such a card will produce a spike dead center in the camera's histogram if the camera's exposure is accurately set. (That's because an 18% gray is a "middle" gray.) You can also adjust the "exposure" during post processing by moving the spike to the center of the software's histogram during post processing. Digital shooters will probably find this card the best to use.

Old wives tale. We have always heard that, but it has always been wrong.

18% is simply not the midpoint of anything, except the human eye's nonlinear response curve. As you mentioned, the printing industry found advantage that it "appeared" to "look" about middle gray to human eyes. But it was 18% reflectivity. And we are not speaking about eyes here.

Which is a moot point in this discussion, because digital cameras have linear sensors, and 18% is simply 18%. 18% is not 50% of anything. It is 18%. In linear data, 18% ought to come out about 18%. 18% is the definition of it. To imagine it should be at 50% is like comparing apples with ... shoe boxes maybe. The same relationship exists.

However, no digital data we ever see is linear. RAW is linear at the sensor, but we cannot see RAW. We have no RAW viewing systems. We only see it after conversion to RGB. Histograms only show RGB. All RGB digital image data we can ever see is gamma encoded. This changes things, unexplained in your story.

The purpose of gamma was to correct the non-linear response of CRT display, starting for early television, and then for computer monitors too. Gamma is typically 2.2, for an exponential math operation affecting 100% of our image data.

18% Gray card = (0.18 ^ 1/2.2) = 0.46, x255 = 117 46% of the histogram scale.

Now 46% at 117 is coincidentally near 50%, which no doubt confuses the troops, but it is not 50%, it is not midpoint of the data, and the reason it is even near is the farthest thing from any definition of midpoint. 18% is not the midpoint of anything in digital data. It is 18%. The actual 50% point of 128 is moved anyway, now up near 187, at 73% of the histogram scale (after being gamma encoded).

As clear and easy and obvious proof, if a photograph is carefully adjusted to put right end of histogram data exactly at 255, but then intentionally underexposed exactly one stop (50%) - we imagine it will move that right end down to midpoint at 128 (in RAW linear data at the sensor. We may expect 128. We do NOT see 128 however. We cannot see RAW. The 128 we see in the histogram is not the midpoint of anything in the data. Any RGB image we ever see is gamma encoded, changing all the numbers.

Simple truth. Explanations should include the facts.

To explain 18% as midpoint of anything is like explaining wearing plaid pajamas keeps the elephants away. Conclusion is coincidentally not so far wrong (we don't see elephants), but the logic is as wrong is it is possible to be.

18% is NOT midpoint of anything in digital data. 18% is 18%.

Sailor Blue
Sailor Blue Forum Pro • Posts: 14,041
Re: Each Has an Advantage Over the Other

Fantasy Photo wrote:

A little history: The 18% gray card was popularized by Kodak for use in the printing trade

Here is another good discussion of the fact that incident light meters, such as the metering system of cameras, are set to read 12% gray, not 18% gray.

http://www.bythom.com/graycards.htm

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